Conventionally, employed in general as an internal pressure test method of a can filled with content such as a low-acid beverage, e.g., coffee and tea into the can and sealed are an internal pressure test method for testing whether an internal pressure is appropriate by measuring a displacement amount in the axial direction of a center panel of a negative-pressure can formed by forming the center panel of a can bottom of a seamless can body into a flat shape and filling content into the seamless can body and sealing the can body or a slightly-positive-pressure can formed by filling and sealing the can after replacing a head space with inert gas such as nitrogen in order to reduce thickness of the seamless can body or an internal pressure test method (called a tapping inspection) for testing whether internal pressure is appropriate by applying an electromagnetic shock to the center panel and measuring a tapping inspection sound (resonance frequency). It is known that, in these internal pressure tests of the can, the test can be carried out with higher accuracy when flatness of the center panel is high. Therefore, in a manufacturing process of the seamless can body, it is an important factor to increase the flatness of the center panel of the can bottom in order to obtain the seamless can body having excellent suitability for the internal pressure tests by the above-described internal pressure test methods and various contraptions for keeping the can bottom flat have been proposed conventionally.
As a seamless can body for positive pressure and having high pressure resistance in spite of use of thin-walled material and an aptitude for the tapping inspection, there is a known seamless can body including, on its can bottom, a rim portion protruding to an outside of the can, an inner wall portion rising from an inner peripheral side of the rim portion, an annular concave portion protruding to an inside of the can, and a flat center panel portion extending continuously from an inner peripheral side of the annular concave portion with a corner portion interposed therebetween (see Patent Document 1).
The flatness of the center panel is defined by a distance of outward or inward bulging of a center portion of the center panel with respect to an outer peripheral end of the center panel as a basis surface (referred to as “bottom sink difference (BSD)”, in general). The BSD is influenced by characteristics of metal materials and various forming conditions and is also influenced, in a case of a seamless can body obtained by drawing/ironing or wall-thinning drawing (stretch drawing)/ironing, by assist air pressure for removing the can body from a punch after the ironing and therefore it is difficult to control the flatness in such a range that the can body can be adapted to the above-described inner pressure test (see Patent Document 2).